A Syrian man holds the revolutionary flag during a demonstration. Regime forces continue to pound the city of Hama. Photograph: AP

A French-led push for a tougher stand on Syria is gathering momentum as regime forces continue to pound the country's fourth city, Hama, in defiance of an ailing UN-sponsored ceasefire.

France has adopted a forward position on the crisis by insisting that it will wait no longer than 5 May before moving to seek a resolution under Chapter 7 of the UN charter to demand the Syrian regime stop its campaign against dissent. Such a move could mean seeking the use of force though it is more likely to demand non-military punitive measures.

Western diplomats have admitted privately however that there is no chance of the UN security council taking action against president Bashar al-Assad as long as he enjoys the support of Russia and China, which have used their vetoes to defend him against sanctions.

The Syrian National Council, the leading exile group, called on the UN to meet in emergency session to discuss the crisis.

Only 15 UN observers have so far made it to Syria from the 300 accepted by Damascus as part of the UN-Arab League plan being promoted by Kofi Annan. Their presence has been unable to stem the violence, which has claimed at least 45 lives each day since a tenuous ceasefire came into effect on 12 April.

Hama, a rebellious city near Syria's border with Lebanon, has borne the brunt of the latest violence with parts of the city reportedly subjected to bombardment and farmlands on the outskirts swept throughout Thursday by elite loyalist units.

A large explosion in a Hama neighbourhood late on Wednesday is thought to have killed close to 70 people. If true, the blast would mark one of the highest single casualty tolls since the Syrian uprising began almost 14 months ago.

State television acknowledged that an explosion had taken place in Hama, but claimed a depot used by rebels to make bombs had blown up, killing 16. Damascus has repeatedly cast the uprising as a two-way fight against armed subversives - a claim that has gradually became a self-fulfilling prophecy as its crackdown against a popular revolt raged on.

The Syrian military remains active in other parts of the country, including the ravaged city of Homs where a large wall is reportedly being built around parts of the Baba Amr district, which was raised by regime forces in February and March.

The outlying areas of Homs, including the satellite town of al-Qusair, have also seen fresh clashes - and claims of savagery that, if proven, would break new ground in the violence. A video posted online in the past few days purports to show a man from Qusair being buried alive as punishment for sending videos to Arab satellite TV channels hated by the regime.

The authenticity of the video could not be immediately verified though urgent attempts were being made to contact residents in Qusair, which has remained largely encircled and isolated for the past three months. Another video circulating on Thursday showed a man in uniform decapitating a prisoner with a large saw.

The military operations are a clear violation of the ceasefire brokered by Annan, who this week acknowledged in a briefing to the UN that he had grave concerns about the regime's willingness to meet its commitments and the ultimate success of his six-point plan.

Annan is next due to report to the security council on 5 May, a day that French Foreign Minister Alain Juppe said would mark a moment of truth for the Syrian regime. ''This cannot continue indefinitely," he said. "We want to see observers in sufficient numbers, at least 300, deployed as quickly as possible. If that does not work, we cannot allow the regime to defy us.''

Europe and the United States have repeatedly baulked at a Libya-style military intervention, though Washington's rhetoric has shifted in recent weeks. US defence secretary Leon Panetta last Thursday told Congress that the US military was prepared to play a role in Syria, but would not act unilaterally.

Key changes in strategic planning positions within the National Security Council have also signalled a more proactive US stance in Syria, where more than 8,000 people have died in an uprising that has increasingly taken the shape of a civil war, which threatens to drag in neighbouring states and stoke the coals of a regional conflict.

Syria's northern neighbour, Turkey, has again warned it would not tolerate a fresh flood of refugees fleeing persecution in Syria. Ankara, like key regional stakeholder Saudi Arabia, has become evermore concerned with the deterioration in Syria, but has not wanted to act alone.

Turkey has twice hinted it would consider establishing a humanitarian corridor near its southern border, an act that could be perceived by Damascus as an act of belligerence.

The UN said it had won agreement from member states for 100 unarmed monitors, as well as additional civilian support staff, to travel to Syria, far fewer than the 300 it is seeking. It blamed the delay on political wrangling in contributing countries.

Norwegian Major General Robert Mood is to be appointed as leader of the UN delegation, the impact of which is drawing comparisons with an abortive Arab League monitoring mission in December that ended in failure.